Fuel cells are alternative energy producing systems that generate electricity from common fuel sources such as natural gas and that, typically, have higher efficiencies and lower emissions than conventional systems. More specifically, fuel cells are electro-mechanical devices that provide electrical power by reacting, for example, hydrogen gas (H2) usually in the form of natural gas or ethanol with an oxidant, e.g., air or oxygen gas (O2). The gases react to produce electrical current and a relatively harmless water bi-product.
For example, a fuel, e.g., hydrogen gas (H2), can be introduced at a first electrode (an anode), where a catalyst encourages production of protons, i.e., hydrogen ions (H+), and electrons (e−) in accordance with the following equation:H2catalyst>2H++2e−
The electrons (e−) are collected in an electric circuit that transmits the electrons to a second electrode (a cathode). Electron flow from the anode to the cathode constitutes usable current, i.e., power. The protons (H+) travel through the electrolyte membrane to the cathode, where, contemporaneously, an oxidant, e.g., air or oxygen gas (O2), is introduced. The oxidant and cathode catalyst react electrochemically with the hydrogen protons and the electrons to produce water and heat in accordance with the following equation:2H++½O2+2e−catalyst>H2O+heat
In addition to high manufacturing cost, the fuel cell industry is faced with several critical challenges that must be resolved before fuel cell systems can be fully commercialized for wide spread power generation applications. These challenges include, without limitation: innovative anode/electrolyte/cathode materials for lower electrochemical losses; durable fuel cell interconnects; improved sealing concepts; compatible metallic interconnects; advanced stack cooling; low-cost fabrication processes; understanding of soot/carbon deposit mechanisms; efficient fuel reformer; and de-sulfurization systems.
There are a number of types of fuel cells, which include, among others, phosphoric acid, proton exchange membrane, molten carbonate, solid oxide, and alkaline. Among the various types of fuel cells, the solid oxide fuel cell (“SOFC”) exhibits many advantages over the other fuel cell systems for power generation. For example, the SOFC has the highest energy efficiency and can tolerate low-cost catalytic materials. Moreover, existing studies indicate that the SOFC system is probably one of the most reliable power generation technologies. Further, the SOFC is best suited for integration with conventional gas turbine engines for improvements in fuel consumption and emission pollution. Most importantly, the SOFC system can operate directly with hydrocarbon fuels, being able to utilize the existing refueling infrastructure fully. Because of these significant advantages, the fuel cell industry has been working diligently to develop compact, efficient, fuel reformers that can effectively convert liquid hydrocarbon fuels into hydrogen-rich gas for SOFC systems used in auxiliary power units.
Liquid hydrocarbon fuels can be reformed to produce hydrogen-rich gas through partial oxidation, steam or auto-thermal reforming. The major requirements for the fuel reformer system used with the SOFC include simple construction, small size and weight, low manufacturing cost, lower operating pressure and temperature, high conversion efficiency, carbon and sulfur tolerance, multi-fuel capability, maximum thermal integration, low maintenance intervals, rapid startup, and acceptable transient response.
A review of the existing fuel processing technologies indicates that most fuel reformers are in the prototype and demonstration stage. In short, current, state-of-the-art fuel reformers are not yet capable of meeting the stringent requirements for commercial or military applications. Particularly, current, state-of-the-art fuel reformers are heavy in weight, large in physical size, and provide only moderate conversion efficiency. Furthermore, most of the fuel reformers are vulnerable to carbon formation, necessitating either frequent cleaning or high oxygen/carbon (“O/C”) ratios for sustained operation. Operating at high O/C ratios, however, reduces the overall system efficiency. Also, the existing catalysts used for the reformers cannot tolerate significant sulfur levels and thus require the liquid fuels to be desulfurized.
Another major difficulty for SOFC reformers germane to the present invention involves the atomization and mixing of liquid fuel with heated air and/or superheated steam. Failure to provide a uniform fuel vapor mixture prior to entering the catalytic reactor can result in hot spots and carbon formation. Moreover, non-uniform gas streams within the mixing chamber and/or catalytic reactor also could lead to significant performance degradation and reduced reformer efficiency. Finally, the catalytic reactor also may encounter a significant pressure drop due to carbon or soot deposits and build-up, which would necessitate additional pressure or momentum to force the gas streams through the catalytic reactor.
Referring to FIG. 1, there is shown a conventional fuel reformer system 10. Typically, a fuel reformer system 10 comprises an integrated fuel injection and mixing system 10 that is connected to a catalytic reactor 3. Ideally, a fuel injector 1 is mounted to or otherwise operatively associated with a mixing chamber 2. The fuel injector/mixing chamber combination supplies a uniform or near uniform fuel vapor mixture to the catalytic reactor 3, which produces a hydrogen-rich gas.
More particularly, liquid hydrocarbon fuel, e.g., natural gas, diesel fuel, jet fuel, gasoline, kerosene or the like, can be supplied to a fuel injector 1, for example, via a control valve 6. To assist fuel atomization, a heated gas stream 4 is simultaneously supplied to the fuel injector, e.g., through a regulator valve 7. Depending on the reformer type, the atomizing gas stream 4 could be either steam flow or heated airflow. For steam- and auto-thermal-type reformers, steam flow is used as the atomizing gas stream 4. Whereas, for partial oxidation-type catalytic reformers, heated airflow is used as the atomizing gas stream 4.
For most fuel reforming applications, it also is preferred that a uniform, secondary fluid flow 5 be supplied to the mixing chamber 2 and, more specifically, the uniform, secondary fluid flow 5 be supplied around the outlet of the fuel injector 1. Providing such a uniform, secondary fluid flow 5 enhances the mixing process and also minimizes liquid fuel droplets from adhering or otherwise attaching to the walls of the mixing chamber 2. A controller 8 can be used to adjust the required flow rates for all three feed streams. More specifically, the controller 8 can control the flow of liquid fuel to the injector 1 by controlling valve 6 and can control the delivery of steam flow or heated airflow to the fuel injector 1 and/or mixing chamber 2 by controlling control valve 7.
To develop a compact, efficient fuel reformer system, it would be desirable to provide an integrated fuel injection and mixing system that can overcome the technical problems enumerated above. It also would be desirable to provide an injection and mixing system that could be easily integrated into various types of fuel reformers. Not only must the new fuel injection system demonstrate better conversion efficiency, it must also be more compact in size with fewer components and lower manufacturing cost. Finally, it would be desirable to provide an injection and mixing system that can demonstrate extended service life without the problem of carbon or coke deposition.